/Engine/Source/Editor/MovieSceneTools/Public/TrackEditors/SubTrackEditorBase.h
This mixin effectively wraps the 2 utility classes above, hooks them up to theISequencerSectioninterface, and implements a few otherISequencerSectioninterface methods. As such, it's mostly handy boilerplate code, which you can avoid (if it doesn't fit your use-case) in favour of using the uility classes above directly.
template<typename ParentSectionClass> class TSubSectionMixin : public ParentSectionClass